The present invention relates to a gas target device for bombarding a gaseous target with charged particles from a charged particle accelerator.
The production of certain radioisotopes requires the irradiation of highly enriched monoisotopic gases with high energy ions at elevated pressure. The greatest possible efficiency of the product is obtained by working with the highest possible intensity of the ion beam current. Since in the eligible range of energies of 10 to 30 MeV per neutron the charged particles have previously quickly lost their energies in solids, it is necessary to provide thin entrance foils as windows at the so-called gas targets. These entrance foils are exposed to the following impacts: pressure, temperature and radiation burden. Experimental experience has shown that rupture or the occurrence of minor leaks can never be completely ruled out.
The entrance window at one end of the chamber of a gas target device through which the accelerated and charged particles enter should be very thin; on the other hand, the gas in the target chamber must be kept at a specified pressure in order to obtain a good irradiation efficiency of the charged particles. In addition, radiation induced destruction of the window during particle irradiation has to be taken into consideration, as already stated. In an exemplary case gaseous xenon-124, 99.8% enrichment, is to be irradiated for six hours with a 30 MeV proton beam in order to obtain iodine-123 as the end product of a known chain of reactions. A great portion of iodine-123 produced in the target chamber is obtained immediately after irradiation.
The problems encountered are due firstly to the very thin film from which the window on the entrance side of the chamber of the gas target must be made and secondly to the fact that the gas chamber must be kept at an internal overpressure which might attain about 15 bar.
Thus, if the already mentioned very thin metal film ruptured during irradiation of the gas target, a considerable amount of radioisotopes produced in the target chamber would escape into the vacuum space of the irradiation apparatus, e.g., a cyclotron, causing contamination of the latter apparatus with the radioactive substances. Moreover, the loss of the enriched target gas would entail high costs.
If a conventional gas target is connected directly with the vacuum system of a beam guide system and/or a particle accelerator, the following drawbacks result in case defects occur at the entrance foils:
1. Losses of the costly gas.
2. Contamination (normally, the irradiated gases have become highly radioactive already after short irradiation periods) of the beam guide system and/or accelerator.
3. Loss of production associated with economic losses.